Semiconductor lasers have an important role as light sources for optical communication systems. By selecting appropriate semiconductor materials, conventionally III-V alloy compounds, lasers which emit in the wavelength range from 0.8 .mu.m to 1.7 .mu.m can be fabricated. At present, long haul optical fiber communication is based on operating wavelengths around 1.55 .mu.m and 1.3 .mu.m, the wavelengths at which single mode optical fibers have minimum attenuation and dispersion respectively. Lasers have advantages over light emitting diodes (LEDs) of providing a coherent, narrow bandwidth source, ideally suited for communications applications. Single mode, narrow linewidth light sources for optical communication are thus dominated by laser diodes fabricated from direct band gap, III-V semiconductor alloy materials, particularly InP/InGaAsP, which emit in this wavelength region. For example, by using low chirp, multiple quantum well (MQW) DFB lasers at 1.55 .mu.m, 2.5 Gb/s transmission systems with a repeater span of 80 km are commercially available, and systems for practical long haul transmission at 10 Gb/s are under development. Shorter wavelength sources, e.g. GaAs/AlGaAs which emits at 0.9 .mu.m, are used for short distance transmission.
The requirement for direct band gap materials, and group III-V semiconductors in particular, for laser sources is, of course, related to the fact that direct recombination of electrons and holes, resulting in radiative emission, occurs with high probability for a no-phonon or .DELTA.k=0 transition. Consequently the early development of LEDs used these materials, particularly GaAs. The conditions for stimulated emission are relatively easily obtained in III-V material by suitable pump means, resulting in development of the first laser diodes in the 1960s. In comparison with Fabry-Perot configurations, advanced laser structures including gain coupled, distributed feedback multi-quantum well structures now provide greater efficiency for narrower linewidth emission, and wavelength selectivity in the range required for communications applications.
Semiconductor lasers from other direct bandgap materials of group II-VI are also being developed, but these devices emit at shorter wavelength than 1.3 .mu.m and currently exhibit poor cw lifetimes.
Nevertheless, the vision of optoelectronics as integrating of optics and electronics onto the same substrate to provide optoelectronic integrated circuits (OEICs) is driving the search for improved and alternative laser light sources.
While GaAs integrated circuits surpass the performance of silicon based ICs for high frequency applications, their use is generally limited to specialized applications where higher performance and speed requirements offset manufacturing issues and relatively high manufacturing costs per unit.
Indeed, with improvements in advanced submicron BiCMOS, heterostructure devices, including SiGe alloys, silicon technology is currently competing with GaAs for some applications in telecommunications in the 1 to 5 GHz frequency range. Silicon technology is particularly attractive if performance requirements can be met, when taking into account the economies of scale in fabrication of high volume, relatively low cost silicon processing.
While hybrid OEICs would combine Si-based electronics and optical materials from direct bandgap III-V and II-VI-based alloys, there remains much speculation and uncertainty about the possibility of all silicon optoelectronics, i.e., monolithics in which each component comprises group IV materials. The doubt arises primarily from the fact that the crystal structures of silicon and other Group IV materials are generally indirect bandgap materials.
From the conventional undergraduate textbook viewpoint, the band structure of bulk group IV materials is characteristic of indirect band gap materials, which means that these materials do not emit light by a direct hole-electron recombination, except in phonon assisted transitions. That is, the .cndot.k=0 condition can be achieved by contribution of a phonon to conserve momentum with much longer lifetime than no phonon transitions which occur in direct bandgap materials. A longer lifetime is an inherent feature of indirect band gap materials, resulting in a lower probability of radiative emission. Thus, indirect bandgap materials, including silicon and silicon-germanium, have long been considered to be incapable of efficient light emission. There is much speculation as to how, if at all, a high efficiency silicon-based light source may be fabricated, let alone whether a laser could be fabricated.
Consequently observation of light emission from Si, SiGe, and SiGeC is a topic of worldwide curiosity. Light emission has been observed in rare earth doped Si and SiGe, strained quantum wells of SiGe, [100] nano-porous Si, nano-porous Ge, isoelectronic impurity doped SiGe, man-made Si columns and dots and nanocrystalline Si and Ge films, and Si quantum wire structures formed by lithography or other means.
Indeed, Japanese Patent Application No. 063310816A, published Nov. 4, 1994, speculates that stimulated emission may be obtained from an optically pumped silicon fine crystal comprising Si atom clusters having a diameter of 1-20 nm.
While the room temperature band gap of silicon is 1.14 eV, and of Ge is 0.67 eV, the band gap of strained Si.sub.1-x Ge.sub.x, alloys spans the range from 1.12 to 0.6 eV, i.e. 1.1 .mu.m to 2 .mu.m. Strain and quantum confinement effects shift optical emission to higher energies in very thin layers (see for example, Rowell et al. J. Appl. Phys. 74(4), p. 2795, Aug. 15, 1993). Thus silicon-germanium alloys are of considerable interest for silicon based optoelectronics for communication systems.
Research in the last five years has demonstrated increasingly efficient light emission from SiGe alloy/Si. One of the present inventors has been involved in the study of photoluminescence (PL) from epitaxial SiGe alloy/Si, being one of the researchers involved in first ever observation of PL from epitaxial SiGe alloy, which had an efficiency of 10% at 4K, as described in Appl. Phys. Letters 57 (10), pp. 1037-1039, Sep. 3, 1990. Low temperature PL was observed in the range 1.2 to 1.7 .mu.m. This group also made the first demonstration of electroluminescence from strained SiGe alloys as described in Appl Phys. Lett. 58(9), Mar. 4, 1991, with emission in the 890 meV region. Electroluminescence from Si/Si.sub.1-x Ge.sub.x multi quantum well pin diodes was observed in the range 1.35-1.55 .mu.m with an internal quantum efficiency of at least 0.1%.
Very recently, Tang et al., in a publication in Electronics Letters Vol. 31, (16), pp. 1385-6, Aug. 15, 1995, reported fabrication and characterization of Si-Si.sub.0.7 Ge.sub.0.3 light emitting diodes of .cndot.60 nm diameter.sup.1, with emission of 1.312 .mu.m, at room temperature, with an estimated internal quantum efficiency of about 3.5%, orders of magnitude higher than their as-deposited SiGe superlattice diode. The luminescence persisted to room temperature with 50% intensity of the 4.2K value. Tang postulated that the nano-fabrication process causes release of the as-grown pseudomorphic strain, which together with the etching induced formation of the SiGe alloy layer on the sidewalls of the dots will distort the original lattice and alter the lattice symmetry inside the small dots, which may help to convert the indirect bandgap to a direct bandgap, causing the improvement of optical efficiency. FNT .sup.1 although these diodes are referred to as `quantum dot" diodes, the term quantum dot structures more typically refers to elements whose dimensions are comparable or less than the confined carrier probability distribution, which is generally less than 200 .ANG.; features with one or more dimensions exceeding 200 .ANG. should therefore not be considered as quantum dots.
All of the approaches for improving radiative efficiency are based on restricting motion of carriers laterally within QWs through artificial micro-structuring or introduction of radiative binding centres.
The achievement of a practical laser diode from a group IV material remains sought after as a significant milestone in technological evolution of optoelectronics.